The invention relates to gate valves and particularly to very large gate valves and the sealing of such valves. While the present invention has particular application to fly ash control systems those skilled in the art will recognize other applications. In a typical application for the present invention the opening through which the fluidized particulate material, the fly ash, is approximately 19 inches in diameter. The overall length in this typical application of the entire valve mechanism is over 100 inches. In a typical application it necessary that the valve tightly seal off flow at up to 15 psig so that maintenance work may be performed down-stream of the valve. Any leakage would be hazardous to personnel working down-stream of the valve.
The prior art includes Steckle U.S. Pat. No. 3,495,805 which shows a gate valve with an eccentric surface that cooperates with a pad on the side of the gate to urge the gate into engagement with a stationery seal. It is inherent in this design that the friction between the eccentric surface and the pad will cause wear of the eccentric surface and the pad.
Whaley U.S. Pat. No. 1,391,9871 discloses another gate valve having dual gates. An eccentric member is disposed between the first and second gates is rotatable to an angular position of greatest eccentricity to force both the gates against stationery seals.
This structure also does not prevent the transfer of lateral forces to the respective gates and thus causes lateral movement of the gates with respect to the seats that cooperate with the gates. The lateral movement of the gate or gates in these structures causes seal wear because of the relative movement between the gate or gates and the seal or seals associated therewith.
Neale U.S. Pat. No. 4,535,801 discloses a gate valve having a pressurized bonnet. The valve shown in this Patent is intended for handling powdered materials, such as powdered coal fed to a steam boiler or coal and lime to a gasifier. Usnick et al. U.S. Pat. No. 4,174,728 also discloses a pressurized bonnet.
Some prior art gate valves have relied on metal to metal seals. While this does eliminate wear problems associated with elastomeric seals, it is not satisfactory for many applications because the metal to metal seal will allow too much leakage particularly at pressures of 14 or 15 psig.
It is an object of the invention to provide a gate valve that will have virtually no seal wear.
Another object of the invention is to provide a gate valve structure that will seal in a manner that results in no lateral movement of the gate.
Still another object of the invention is to provide a mechanism for latching the gate against the seal that will maintain the sealing relationship even if fluid pressure that operates the latching mechanism fails.
Yet another object of the invention is to provide apparatus that will seal in a manner that will minimize leakage and hazards to personnel working down stream from the valve.
Another object of the invention is to provide a structure in which the seal is disposed at a location which is not in the path of direct material flow when the valve is open.